1. Field of the Invention
The present invention relates to a process for laser welding resinous members, an apparatus for the same, and a laser-welded resinous product. More specifically, it relates to a process for integrally bonding transparent resinous members exhibiting transmissivity to laser beams and absorptive resinous members exhibiting absorptivity to laser beams by laser welding, an apparatus for the same, and the resulting laser-welded resinous products.
2. Description of the Related Art
Recently, from the viewpoint of weight saving and cost reduction, it has been carried out extensively to resinify component parts of various fields, such as automobile component parts, thereby making them into resinous molded products. Moreover, from the perspective of producing resinous molded products with high productivity, it is often the case to employ the following measures. A resinous molded product is molded as a plurality of the component parts separately in advance. Then, the resulting independent component parts are bonded with each other.
Laser welding processes have been utilized conventionally in order to bond a resin with another resin. For example, Japanese Unexamined Patent Publication (KOKAI) No. 11-348,132 discloses a laser welding process. In the conventional laser welding process, a transparent resinous member exhibiting transmissivity to a laser beam is overlapped on an absorptive resinous member exhibiting to absorptivity to the laser beam. Thereafter, the transparent resinous member is irradiated with the laser beam. Thus, the transparent resinous member and absorptive resinous member are heated to melt at the surfaces, thereby bonding them integrally.
In the conventional laser welding process, the laser beam is absorbed at the interface of the absorptive resinous member when it transmits through the transparent resinous member and reaches the interface. The laser beam absorbed at the interface is accumulated as energy. As a result, the interface of the absorptive resinous member is heated to melt, and simultaneously the interface of the transparent resinous member is heated to melt by the heat transfer from the interface of the absorptive resinous member. When the interface of the transparent resinous member and the interface of the absorptive resinous member are applied to each other under the circumstance, it is possible to bond them integrally.
However, it is not necessarily possible to obtain a uniform welded state by the above-described conventional laser welding process in certain cases. Let us consider the case of welding a resinous product, a box-shaped container, by the conventional laser welding process with reference to FIG. 18. For instance, a box 4 has an opening molded from an absorptive resin, and is welded to a cover 3 composed of a transparent resin. The box 4 and cover 3 are welded by using a laser beam 1 along a weld line 2 to complete a welded product 5. When scanning the linear parts of the weld line 2 with the laser beam 1 to weld thereat, the laser energy per unit time is constant on the surface 4′ of the box 4 under the conditions that the laser power, the scanning speed and the thickness of the cover 3 are constant. Accordingly, it is possible to weld uniformly in the longitudinal direction and widthwise direction of the weld line 2 in welding at the interface between the cover 3 and the box 4. However, the laser beam 1 has a finite irradiation cross-sectional area. Consequently, it is not necessarily possible to weld uniformly at the curved parts 2′ of the weld line 2 (i.e., parts adjacent to the corners of the welded product 5) at which the scanning direction of the laser beam 1 is changed from one direction to another.
Moreover, FIG. 17 illustrates a relationship between a welding line 2 and a laser-beam spot 3 adjacent to a corner of the welding line 2 when a laser beam has an ellipse cross section and a spot diameter equal to the width of the weld line 2. Under the condition that the scanning speed of the laser beam is constant, the travel distance of the laser beam along the left-hand side tangent “A” of the laser-beam spot 3 forming the weld line 2 is equal to the travel distance of the right-side tangent “B” thereof at the linear parts of the weld line 2; but the travel distance of the laser beam along the major-curvature-radius curved part R2 (or along the outermost peripheral line “A”) is longer than the travel distance of the laser beam along the minor-curvature-radius curved part R1 (or along the innermost peripheral line “B”) at the curved part of the weld line 2. This implies that, when the energy of the laser beam irradiating per unit time is equalized at the locus 1 of the center of the laser spot 3 (shown with the alternate long and short dash line of the drawing), the energy is insufficient at the outermost peripheral line “A”; but it is excessive at the innermost peripheral line “B.” Thus, the energy density has become higher at the minor-curvature-radius curved part R1 than at the major-curvature-radius curved part R2 in the curved part of the weld line 2. As a result, the curved part of the welding line 2 is inferior to the linear parts thereof in terms of the weld strength. Moreover, the weld might be insufficient at the major-curvature-radius curved part R2, and the constituent resin might be degraded at the minor-curvature-radius curved part R1 because of the excessive energy.